Solar energy collection systems using compound parabolic concentrators (CPC's) are known and are described in my U.S. patent application Ser. No. 902,381 filed May 3, 1978 (now U.S. Pat. No. 4,198,955), and its equivalent Canadian Pat. No. 1,047,343, the disclosure of which is incorporated herein by reference.
A CPC collector is characterized by a compound parabolic shape which concentrates solar energy received through the mouth of the parabola onto a tube through which fluid to be heated is passed. The parabolic shape is determined by the relationship of the concentration ratio (C), i.e., the ratio of the transverse width of the mouth to the outer circumference of the tube, to the acceptance angle (.theta.), which is the angle to the axis of the parabola within which light rays are concentrated onto the tube. The relationship is EQU C=(1/sin .theta.)
In my aforementioned Canadian Patent, there is described a solar energy collection system which is comprised of modular solar collectors. Each module comprises a plurality of elongate envelopes physically joined together in fixed immovable relationship to each other in an integral body structure and a transparent cover sealingly joined to the body structure. Each envelope is evacuated and has a reflective inner surface. A tube having a selectively absorbing surface for selectively absorbing energy having predetermined wavelengths passes sequentially through the envelopes. The tube conveys fluid to be heated to the module and removes heated fluid from the module.
Although the modular construction is satisfactory, difficulties have arisen in constructing an integral body structure of the type described in the aforementioned patent, especially of vitreous ceramic material. One aspect of the invention is concerned with an improved form of body construction for the module.
A CPC-type collector is highly efficient in collecting solar energy and in heating fluid passing through the tube. Problems arise, however, when the thermal demand on the system is satisfied, both in the short and in the long term, or when stagnation occurs as a result of loss of transport fluid, pump failure, power failure, limited thermal storage capacity and human error, at which time it becomes necessary to "switch-off" the collector. If such shut-down did not occur, irrevocable damage to the selective coating of the absorber tube could occur and an undesirable pressure rise in the fluid system may result. A second aspect of the invention provides a mechanism for obviating these problems.